In CT X-ray imaging of a patient, X-rays are used to image internal structure and features of a region of the person's body. The imaging is performed by a CT-imaging system that images internal structure and features of a plurality of contiguous relatively thin planar slices of the body region using X-rays.
The CT-imaging system generally comprises an X-ray source that provides a planar, fan-shaped X-ray beam and an array of closely spaced X-ray detectors that are coplanar with the fan beam and face the X-ray source. The X-ray source and array of detectors are mounted in a gantry so that a person being imaged with the system, generally lying on an appropriate support couch, can be positioned between the X-ray source and the array of detectors. The gantry and couch are moveable with respect to each other so that the X-ray source and detector array can be positioned axially at desired locations along the patient's body. In addition, the gantry, or X-ray source supported in the gantry, is rotatable around the axial direction so that the X-ray source can be positioned at desired angles, referred to as “view angles”, around the patient's body.
To image a slice in a region of a patient's body, the X-ray source is positioned at the axial position of the slice and the X-ray source is rotated around the slice to illuminate the slice with X-rays from a plurality of different view angles. At each view angle, detectors in the array of detectors measure intensity of X-rays from the source that pass through the slice. The intensity of X-rays measured by a particular detector in the array of detectors is a function of an amount by which X-rays are attenuated by material in the slice along a path length from the X-ray source, through the particular slice, to the detector. The measurement provides information on composition and density of tissue in the slice along the path-length.
For example, if intensity sensed by an “n-th” detector in the array of detectors when the X-ray source is located at a view angle θ is represented by I(n,θ) then I(n,θ)=IOexp(−∫μ(l)dl). In the expression for I(n,θ), IO is intensity of X-rays with which the X-ray source illuminates the slice, integration over l represents integration over a path through material in the slice along a direction from the X-ray source to the n-th detector and μ(l) is an absorption coefficient for X-rays per unit path-length in the material at position l along the path. (Dependence of the integral on n and θ is not shown explicitly and is determined through dependence of the length and direction of the path-length l on n and θ.)
From IO and the sensed I(n,θ) an amount by which X-rays are attenuated along path-length l and a value for ∫μ(l)dl can be determined. The attenuation measurement provided by the n-th detector at the view angle θ therefore provides a value for the line integral of the absorption coefficient along a particular path length through the slice which is determined by θ and the known position of the n-th detector relative to the X-ray source.
It is convenient to represent the line integral, hereinafter referred to as an “absorption integral”, of the absorption coefficient along a path through a slice by the symbol A(n,θ,z) so that A(n,θ,z)=∫μ(l)dl. In the expression for A(n,θ,z), z represents an axial coordinate of the slice as measured along a z-axis of a coordinate system for which the z-axis is coincident with the axial direction around which the X-ray source rotates. For convenience, an x-axis and y-axis of the coordinate system are assumed to be horizontal and vertical axes respectively and view angle θ is an azimuth angle about the z-axis measured relative to the y-axis. At a view angle of 0° the X-ray source is directly above the patient and X-rays from the X-ray source pass through the patients body from front to back, i.e. in an anterior-posterior direction. At a view angle of 90° the X-ray source is at a side of the patient and X-rays from the X-ray source pass through the patient from one side to the other of the patient's body, i.e. in a lateral direction.
The set of attenuation measurements for a slice provided by all the detectors in the detector array at a particular view angle θ is referred to as a view. The set of attenuation measurements from all the views of the slice is referred to as a “projection” of the slice. Values for the absorption integral A(n,θ,z) provided by data from the projection of the slice are processed using algorithms known in the art to provide a map of the absorption coefficient μ as a function of position in the slice. Maps of the absorption coefficient for the plurality of contiguous slices in the region of the patient's body can be used to provide a three dimensional map of the absorption coefficient for the region. The map is used to display and identify internal organs and features of the region.
In some CT systems, to image a region of a patient, the patient is moved stepwise in the z direction to “step” the region through the gantry that houses the X-ray source and detector array. Following each step, the X-ray source is rotated through 360 degrees or (180+Δ) degrees, where Δ is an angular width of the fan beam provided by the X-ray source, to acquire a projection of a slice of the region. In some CT systems a “spiral scan” of a patient is performed in which the region of the patient is steadily advanced through the gantry while the X-ray source simultaneously rotates around the patient and projections of slices in the region are acquired “on the fly”.
For safety and health reasons it is desirable to minimize a dose of X-ray radiation that a patient receives when the patient is imaged using a CT-system. However, a signal to noise ratio (SNR) of an X-ray intensity measurement provided by a detector in the CT-system decreases as a number of X-ray photons reaching the detector decreases. Therefore, if intensity IO of X-rays used to acquire a CT-image is too low or attenuation of the X-rays after passing through the patient is too high, accuracy of absorption measurements decreases and quality of a CT-image provided by the CT-system is degraded.
Various methods are known in the art for reducing radiation exposure of a patient to X-rays during acquisition of a CT-image of a region of the patient's body without unduly compromising quality of the CT-image. The methods generally involve modulating IO of the X-ray source that provides the X-rays, in accordance with an appropriate function, so that IO is greater for views for which attenuation of X-rays is relatively high and smaller for views for which X-ray attenuation is relatively low. The function used to define IO as a function of X-ray source position is hereinafter referred to as an “X-ray modulation function”.
In prior art CT-systems, data for generating an appropriate X-ray modulation function for imaging a region of a patient's body is often acquired from two “reconnaissance” scans, hereinafter referred to as “planar scans”, of the region that are performed prior to imaging the region. The planar scans are used to determine a maximum attenuation, hereinafter referred to as a “peak attenuation”, for X-rays in each of two generally orthogonal views for each slice of the region to be imaged. (A maximum attenuation for a view is defined as a maximum for the expression 1/exp(−∫μ(l)dl), or equivalently, a maximum for exp(∫∞(l)dl), for the view.) The attenuation maxima in the two views are used to determine minimum intensities at which to expose the views in order to acquire attenuation data at a desired SNR.
Usually, in a first planar scan, a first view of each slice is taken at 0°, i.e. an anterior-posterior view, and in a second planar scan a view of each slice is taken at 90°, i.e. a lateral view. For one of these views a longest path length in the view is generally longer than the longest path length in any other view of the slice. For the other of these views a longest path length in the view is generally shorter than the longest path length in any of the other views of the slice. (For example, for a slice in the region of the chest the path lengths are longest for the lateral view angle and shortest for the anterior-posterior view. For a slice in the region of the head, path-lengths are longest for the anterior-posterior view and shortest for the lateral view.)
The larger of the peak attenuations determined from the two “planar” views is therefore generally larger than the peak attenuations for views of the slice at any other view angle. Similarly the smaller of the peak attenuations determined from the planar views is smaller than the peak attenuations for any other view angle of the slice. The peak attenuations from the planar views “bracket” the peak attenuations for views of the slice. If peak attenuation in a view of a slice at view angle θ is represented by PA(θ,z) for a slice at coordinate z then either PA(0°,z)≦PA(θ,z)≦PA(90°,z) or PA(90°,z)≦PA(θ,z)≦PA(0°,z).
As a result, minimum X-ray intensities required to acquire data at the two views for a desired SNR bracket minimum X-ray intensities required at any of the other views of the slice that are required to acquire the other views at the desired SNR. Furthermore, attenuation data as a function of view angle is periodic with period of 180° and generally, peak attenuation value is a smooth function of view angle. Therefore, often, the peak attenuations from the planar views are used to determine an X-ray modulation function that varies harmonically as a function of view angle and has maximum and minimum values that are proportional respectively to the maximum and minimum peak attenuations of the planar views.
It is noted that whereas data from planar scans are used to determine modulation functions for reducing overall exposure of a patient to X-rays the planar scans themselves expose a patient to X-rays. It is also noted that irrespective of performing two planar scans for use in generating modulation functions, in general at least one planar scan of a patient is performed prior to the patient being CT-imaged to acquire data for determining position of a region of the patient to be CT-imaged.
U.S. Pat. Nos. 5,379,333, and 5,400,378 the disclosures of which are incorporated herein by reference describe acquiring “planar” data for a slice by imaging the slice at 0° and 90°. The planar data is used to determine a maximum value for IO for the slice and an X-ray modulation function that varies harmonically as a function of view angle.
U.S. Pat. No. 5,822,393 describes a method of adjusting a value for IO for a slice using a predicted maximum value for attenuation of X-rays in a projection of the slice. The predicted maximum attenuation is determined using a maximum attenuation measured from a projection for at least one preceding slice. The predicted attenuation is used to determine a value for IO for which, after IO is attenuated by the predicted maximum attenuation, a number of X-ray photons reaching an X-ray detector is greater than quantum noise of the detector.